Chemistry Reference
In-Depth Information
[O]
[O]
[O
]
[R
R O
2
H
R O
3
H
SOH]
R H
Sulphinic Acid
Sulphonic Acid
H
2
O
[O]
H
2
O
H
2
O
2[O]
H
2
O
-RSH
-RSH
-RSH
O
O
O
[O]
2[O]
[O]
RSSR
RSSR
RSSR RSSR RSS
R
O
O
O
O
Thiosulphinate
Thiosulphonate
Disulphone
Fig. 11.41
Oxidation of thiols.
sulphines (>C=S=O) are oxidised by MTO/H
2
O
2
to
ketones and SO
2
[282]. Sulfur monoxide is a tran-
sient intermediate in the latter reaction, formed by
fragmentation of sultine (the 'epoxide' of the
sulfine). The sulfine itself can be produced in high
yield in a few minutes at 25°C.
H
2
O
2
C S
-
Na
+
R
2
N
C SS C
NR
2
2R
2
N
S
SS
Fig. 11.42
Dithiocarbamate oxidation to thiuram disulfide.
4.4 Halogenations
H
2
O
2
WO
4
2
HOCH
HOCH
2
CH
2
SH
2
CH
2
SO
3
H
The general utility of hydrogen peroxide for the con-
trollable in situ generation of halogens other than
fluorine from the corresponding halogen acid has
been discussed in Section 2 and earlier in this
section. Bromine probably offers the most advan-
tages when generated in this way, from both cost and
convenience viewpoints: from the atom utilisation
standpoint, complete incorporation of Br in the
product scores heavily over generation of HBr by-
product for external recycling. Halogenation also
provides a means of side-chain oxidation of aro-
matics, as outlined earlier. Sometimes the benzylic
halides themselves are the desired products, as in
the example shown in Fig. 11.45, where 4¢-
bromomethyl-biphenyl-2-carbonitrile is made from
4¢-methyl-biphenyl-2-carbonitrile by side-chain
bromination with H
2
O
2
/HBr, activated by light or the
radical initiator AIBN, refluxing in cyclohexane for
1.5 h [283].
Nuclear bromination is exemplified by the pro-
duction of tetrabromo-bisphenol A using H
2
O
2
/HBr
[284], one of a number of flame-retardant additives
that can be made in this way. In halogen substitu-
tions such as these, essentially all of the halogen can
be put into the product, instead of generating a sto-
ichiometric amount of HX that must be treated
externally.
Fig. 11.43
Isethionic acid production from 2-thioethanol.
In a recent careful study, Espenson [280] oxidised
symmetric disulfides by H
2
O
2
/MTO in acetoni-
trile/water. Thiosulfinates RS(O)SR can be made in
near-quantum yield, or oxidised via thiosulfonate
RS(O)2SR to the sulfonic acid. The thiosulfinate also
can disproportionate to disulfide and thiosulfonate.
In this study, the mono-Re and diperoxo-Re species
were found to make similar contributions to the
catalysis. This chemistry also should be accessible via
other peroxo-metal systems (V, Mo, W).
S-X Oxidations
In addition to disulfides, sulfenamides are made
commercially as rubber vulcanisation accelerators,
and these arise from oxidative coupling of thiols
and (usually secondary) amines [281] (Fig. 11.44).
Again, this is currently an important industrial use
of H
2
O
2
in chemical synthesis.
Carbon-sulphur bonds are cleaved oxidatively by
peroxides. For example, thioketones (>C=S) and
